Covalent attachment of the hydrophilic polymer poly(ethylene glycol), abbreviated PEG, also known as poly(ethylene oxide), abbreviated PEO, to molecules and surfaces is of considerable utility in biotechnology and medicine. In its most common form, PEG is a linear polymer terminated at each end with hydroxyl groups:HO—CH2CH2O—(CH2CH2O)n—CH2CH2—OH
The above polymer, alpha-, omega-dihydroxylpoly(ethylene glycol), can be represented in brief form as HO-PEG-OH where it is understood that the -PEG- symbol represents the following structural unit:—CH2CH2O—(CH2CH2O)n—CH2CH2—where n typically ranges from about 3 to about 4000.
PEG is commonly used as methoxy-PEG-OH, or mPEG in brief, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification. The structure of mPEG is given below.CH3O—(CH2CH2O)n—CH2CH2—OH
Random or block copolymers of ethylene oxide and propylene oxide, shown below, are closely related to PEG in their chemistry, and they can be substituted for PEG in many of its applications.HO—CH2CHRO(CH2CHRO)nCH2CHR—OHwherein each R is independently H or CH3.
To couple PEG to a molecule, such as a protein, it is often necessary to “activate” the PEG by preparing a derivative of the PEG having a functional group at a terminus thereof. The functional group is chosen based on the type of available reactive group on the molecule that will be coupled to the PEG. For example, the functional group could be chosen to react with an amino group on a protein in order to form a PEG-protein conjugate.
PEG is a polymer having the properties of solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule “conjugate” soluble. For example, it has been shown that the water-insoluble drug paclitaxel, when coupled to PEG, becomes water-soluble. Greenwald, et al., J. Org. Chem., 60:331-336 (1995).
The prodrug approach, in which drugs are released by degradation of more complex molecules (prodrugs) under physiological conditions, is a powerful component of drug delivery. Prodrugs can, for example, be formed by bonding PEG to drugs using linkages which are degradable under physiological conditions. The lifetime of PEG prodrugs in vivo depends upon the type of functional group linking PEG to the drug. In general, ester linkages, formed by reaction of PEG carboxylic acids or activated PEG carboxylic acids with alcohol groups on the drug, hydrolyze under physiological conditions to release the drug, while amide and carbamate linkages, formed from amine groups on the drug, are stable and do not hydrolyze to release the free drug.
Use of certain activated esters of PEG, such as N-hydroxylsuccinimide esters, can be problematic because these esters are so reactive that hydrolysis of the ester takes place almost immediately in aqueous solution. It has been shown that hydrolytic delivery of drugs from PEG esters can be favorably controlled to a certain extent by controlling the number of linking methylene groups in a spacer between the terminal PEG oxygen and the carbonyl group of the attached carboxylic acid or carboxylic acid derivative. For example, Harris et al., in U.S. Pat. No. 5,672,662, describe PEG butanoic acid and PEG propanoic acid (shown below), and activated derivatives thereof, as alternatives to carboxymethyl PEG (also shown below) when less hydrolytic reactivity in the corresponding ester derivatives is desirable.PEG-OCH2CH2CH2CO2H                PEG butanoic acidPEG-O—CH2CH2CO2H        PEG propanoic acidPEG-O—CH2CO2H        carboxymethyl PEG        
In aqueous buffers, hydrolysis of esters of these modified PEG acids can be controlled in a useful way by varying the number of —CH2— spacers between the carboxyl group and the PEG oxygen.
There remains a need in the art for further methods of controlling the hydrolytic degradation of activated polymer derivatives.